The present invention pertains generally to wind turbine systems and more particularly to high-tip-speed-ratio wind turbine systems for producing electrical power economically. An important factor in designing a wind turbine system for generating electrical power is the economy of such a system. This includes not only efficiency of operation of the device but also capital costs of producing the system and the reliability and safety factors which accompany operation. Reduction of mechanical complexity in wind turbine systems reduces overall cost and provides greater reliability during operation.
Generally, two types of wind turbines have been used in the past to harness wind energy for various purposes. The two types of wind turbines include the fixed-pitch wind turbine and the variable-pitch or variable-geometry wind turbine.
The variable-pitch or variable-geometry wind turbine has been used in the prior art both to control rotational speed of the wind turbine and to increase the amount of energy extracted from the wind. Control of rotational speed using variable-pitch wind turbines has been used in some prior art devices to provide a constant rotational speed for a wide range of actual wind speeds which allows an alternator coupled to the wind turbine to provide a fixed output frequency and voltage. Not only do such devices fail to extract an optimum amount of energy from the wind, the variable-pitch, variable-geometry wind turbine is expensive, requires complex mechanical devices for proper operation, and failure of the complex mechanical controls can cause high-rotational-speed-induced failure in high winds which can cause extensive damage to equipment and possible injury to individuals. Thus, for optimum reliability, simplicity, and cost, it is desirable to use a fixed-pitch or fixed-geometry wind turbine if such a device can be made to operate in an efficient manner.
Of the fixed-pitch type wind turbines, there are two types of wind turbines, the multivane wheel and the high-tip-speed-ratio wind turbine. The multivane wheel turbine device, which has typically been used on farms to pump water, etc., has a relatively large number of blades having a large pitch angle relative to the plane of rotation. The multivane wheel turbine is therefore able to create a considerable amount of torque at relatively low rotational speeds, which is necessary for operating certain types of machinery.
In contrast, the high-tip-speed-ratio wind turbine has a small number of blades, usually 2-4 blades, which are shaped similar to a mirror image of airplane propeller blades. The pitch of the blade tips relative to the plane of rotation is much less than the pitch of the blades of the multivane wind turbine. Because of these differences, the high-tip-speed-ratio wind turbine operates at a considerably higher speed than the multivane wind turbine for a given wind velocity.
To provide maximum energy output from the wind turbine device, it is desirable to operate at maximum torque for the particular wind speed available. Maximum torque of the high-tip-speed-ratio wind turbine occurs at maximum aerodynamic lift. Maximum aerodynamic lift occurs when the propeller blades have an optimum angle of attack with the relative wind vector which is a combination of the actual wind vector and the wind vector resulting from propeller blade motion, i.e., the blade motion wind vector. Consequently, for a given actual wind velocity, a predetermined rotational speed exists at which the propeller blade is at an optimum angle of attack, producing maximum lift and maximum torque from the wind turbine, which in turn allows for maximum extraction of energy from the wind turbine system. Therefore, although the fixed-pitch wind turbine is more desirable from the standpoint of cost and reliability, the fixed-pitch wind turbine often does not provide optimum output power since it operates most efficiently only at its design pitch.
Several prior art devices have attempted to overcome this problem by matching the load of the wind turbine to its output. For example, U.S. Pat. No. 3,974,395 issued Aug. 10, 1976 to Bright, U.S. Pat. No. 4,095,120 issued June 13, 1978 to Moran et al., and U.S. Pat. No. 4,205,235 issued May 27, 1980 to Pal et al. disclose use of electronic circuits to control the current of field windings in generators and alternators and the addition of discrete loads in order to maintain maximum efficiency. The primary problem with these devices is that the electronic control circuitry operates in accordance with the rotational speed of the wind turbine. Of course, maximum efficiency can not be achieved strictly by sensing the rotational speed of the wind turbine since the wind turbine can be made to rotate at the same rotational speed for a wide range of actual wind speeds due to aerodynamic stall, thereby providing less than maximum power output. Additionally, these prior art devices vary load to the wind turbine by changing the amount of current to the field windings of a rotating excited field alternator or generator, both of which require slip rings or commutators which can wear or fail, thereby causing a high-rotational-speed-induced failure of the wind turbine. Moreover, these devices do not disclose an efficient manner of utilizing the electrical power generated.
U.S. Pat. Nos. 4,274,010 and 4,280,061 issued June 16, 1981 and July 21, 1981, respectively, to Lawson-Tancred disclose matching of power available from the wind to power delivered to the load using fixed-pitch wind turbines. Both of these devices, however, require mechanical safety devices in very high winds. Excessive complexity of these types of failure proof mechanisms increases cost. Moreover, the devices disclosed by Lawson-Tancred require conversion of energy to an intermediate form, i.e., hydraulic storage which further increases capital costs and results in significant decreases in efficiency.
U.S. Pat. No. 2,230,526 issued Feb. 4, 1941 to Claytor discloses another device which matches power available from a fixed-pitch wind turbine to the load. In the Claytor device, a wind driven generator is connected to an electric motor of larger capacity which operates a water pump. The nature of the interconnection of the generator and electric motor ensures that as power in the wind increases, so does power delivered to the load, until a point is reached where the turbine cannot supply sufficient power to the load under normally strong winds.
However, the Claytor device requires the use of a closely matched generator/motor combination which significantly increases capital costs of the system. Moreover, the motor transforms the electrical power generated by the generator to an intermediate form, i.e., mechanical energy which must then be used to perform some operation such as pumping water which further decreases system efficiency. An unlimited supply of water must also be provided with such a system. Of even more importance, the combined use of a generator and and motor in the Claytor device is capable of only preventing a high-rotational-speed-induced failure in normally strong winds. In abnormally strong winds, catastrophic failure is likely. Additionally, excessive wear or lack of maintenance of the generator brushes, failure, low charge, or freezing of the battery, or leakage or lack of water supply to the pump would also result in a high-rotational-speed-induced failure of the system.